Many diverse enzymes interact with phosphate compounds which can act as substrates, cofactors or allosteric inhibitors. Although these enzymes encompass a wide spectrum of activities, they all share an ability to recognize the phosphate group. Until now, no general procedure has been available which permitted the identification and localization of such sites. Ferrate ion, a highly active analog of orthophosphate, appears on the basis of extensive preliminary studies to hold good promise as a probe for such groups and should permit identification of phosphate sites by virtue of its ability to modify essential amino acid residues in such sites. Two intensively studied enzymes which interact with phosphate compounds, E. coli alkaline phosphatase and rabbit muscle phosphoglucomutase which are inactivated by ferrate, have been selected for the identification of the altered amino acid. The study will provide both for testing and development of the procedure and an opportunity to verify the participation of the active serines known to be present in these enzymes and to see if other amino acids are involved. In the case of phosphoglucomutase, the study should aid in identifying the site where the glucose monophosphate substrate is bound. Representatives of the several enzyme clases which interact with phosphate will be screened to test the applicability of the procedure. A selection of non-phosphate interacting enzymes will be tested as negative controls. The successful development and application of the proposed procedure should greatly elucidate the function and control of the many phosphate-interacting enzymes which occur in the living cell.